1. Field of the Invention
The disclosed invention relates to process improvements, methods, and equipment that is adapted to successfully laminate a metal strip, in particular, steel or aluminum strip, with a thermoplastic film based on a polymer such as polyethylene, polyester, polycarbonate, vinyl, kynar, acrylic, or polypropylene in an economical commercial operation. The invention relates to particular technical aspects of operating an economically viable commercial batch coating line for laminating coils of flat metal substrates.
2. Discussion of the Related Art
Existing commercial coil paint coating lines are generally continuous in nature and consist of long processing lines with multiple processing steps. A considerable amount of equipment is dedicated to the required material handling for a continuous process. In addition to reels, there is an entry and exit looping tower so the line can continue the coating operation while the new coil is spliced to the end of the previous coil. There are no commercially practical splicing methods for a moving strip, so strip storage towers are required to allow the entry end of the line to stop while the coil ends are joined. Looping towers include tension controlling equipment, guiding equipment, and bridle roll equipment. This makes a continuous processing line very long, complicated, and expensive.
Existing commercial paint lines require higher line speeds for favorable economic operation. Since a large work crew is required to operate a complicated, high production line, economic considerations require that the line speed be as high as possible. Paint lines operate as fast as practical, very commonly at speeds 200 to 450 fpm. The speed is normally restricted by the length of drying oven and the time to cure the paint.
Because commercial coil paint lines have the most favorable economy of scale with a large production order, it is unattractive to economically coat small orders. Paint lines commonly charge a premium for painting a one or two coil order due to the higher costs associated with switchover and cleanup time.
It is a difficult technical and operational challenge to coat small orders with economics that allows for practical competition with the large coil paint lines.
Existing laminating lines have been put into service utilizing similar production and economic planning as commercial paint lines. A number of coil paint lines include a laminating unit after the second drying oven. The lamination step utilizes an adhesive that is painted on the strip surface and dried in an oven. The film is then pressed onto the adhesive in a continuous operation. The strip has a laminated film on one side and the other side is painted or left uncoated. Similar issues of economics of scale become part of the commercial operation of a laminating operation. A number of operators are required to run the line, staff the warehouse, staff the front office, etc. which require a high operating speed for economic efficiency. Laminating speeds less than 100 fpm become economically unattractive.
Due to current economics, laminating has not presently replaced paint coatings for most products that are pre-painted in the coil form. In general, the films are designed to be utilized with pressure adhesives and are often have special printing to provide important aesthetics or cosmetic appearance. Economic film coatings have not been a serious pursuit and are considered expensive compared to paint.
Thermal lamination of a thermoplastic film on a metal substrate is an attractive alternative to current production methods just described. For example, U.S. Pat. Nos. 5,318,648, 5,238,517, 5,093,208, 5,059,460, 4,980,210, and 4,957,820 by Heyes et al, and U.S. Pat. Nos. 6,217,991 and 6,200,409 by Tanaka et al., and U.S. Pat. Nos. 6,758,903 and 5,919,517 by Levendusky et al. describe certain technical features of a pilot thermal laminating process and experimental film materials. In these examples, various laminating process steps, parameters, and film types are disclosed which are applied to a metal substrate. However, operating parameters and requirements of a commercial production line, which consider important business variables such as capital needs, labor, utilities, etc., are not described in these patents. For example, even though the figures in Levendusky et al. show a reel to reel laminating process, many important features and methods required for a convenient, commercially operable, and financially profitable production facility are not taught or disclosed.
In particular, a number of important design, technical, and operational problems must be solved for the commercial viability of a heat based, batch laminating production line. The potential use of thermoplastic films is economically appealing and provides important environmental and energy benefits. A coating industry has not grown up to exploit the economic advantages of thermoplastic films due to the technical, business, operational, and market issues still to be resolved.
One important operating issue for a thermal based laminating line is an economical design for a small production level, about 5-15% of a commercial continuous coil paint line. The problems of scaling a coating operation down to a small production level with only the essential processing steps are raised. From a business standpoint, it is desirable to begin production of a new coating method with a low capital entry point and, simultaneously, low operating costs. It is very important to reduce capital expenditures by avoiding the need for process line material handling equipment, such as the looping towers previously described. Thus, it is desirable to operate in a batch mode, i.e. one coil at a time. And also an order of a single coil.
The term “batch operation” can be confusing in regard to a coil coating line. When a coil is threaded on a batch production line, there is a long time period where the line is operated in a continuous and steady state manner. For the purposes of this invention, the term “batch operation” means that coils are coated in sequence and the coating portion of the line is started and stopped for each coil. A batch operation may include more than one pay off reel, or may include more than one winding reel.
One important operational consideration is how the film width is matched to the substrate width. Existing laminating operations using pressure sensitive adhesives use films widths which match the metal substrate width. During the laminating step, the position of the film or the position of the strip is guided so that the laminated film is applied correctly. There has not been any practical method disclosed where standard film widths may be utilized for economical film purchases.
It is a distinct commercial advantage to provide for utilizing a thermal laminating film with standard sizes rather than custom order each individual film roll to match the metal substrate. Film pricing is better with improved production scheduling and lot sizing at the film supplier. Also, production is better if freed from the time delays required to order in a particular film width. This requires that an acceptable method of dynamically matching the film width to the metal substrate is performed on the laminating production line.
It is well known in the art to trim a composite laminate film structure after laminating by using fixed position trimming knives. Various stationary and rotating knives are used which are locked at a fixed width. Both the film and the substrate, such as paper, are then trimmed together to the final width. The equipment needed to dynamically trim the edges of a wider film from a thick metal substrate has not been taught or disclosed. For example, U.S. Pat. No. 6,732,625 by Boynton, et al., U.S. Pat. No. 5,058,475 by Tidland, et al., and U.S. Pat. No. 5,125,301 by Miller, et al. describe methods of moving rotating knives to a new width position only while the knife is not cutting.
It is economically preferable to side trim excess film width rather than the entire film-metal structure due to the relative cost, and also due to the complexity of the needed equipment. The equipment and methods of trimming a film are easier to implement than side trimming a metal substrate. It is not desirable to trim both the metal and film to the correct width after laminating, nor is it desirable to let the excess film width simply overhang the metal edge. The overhanging film has a tendency to fold over on top of the metal, causing a severe coil winding defect.
Another important consideration is film shrinkage during heat laminating. When the cold film is applied to a heated metal substrate, it expands due to the temperature change. When the laminate is rapidly cooled, the film width changes more than the width of the metal substrate which exposes the edges of the metal. Film shrinkage can also occur due to crystallinity changes when an oriented film is heated and cooled. It is not attractive to order a film width for the metal substrate and account for multiple factors that affect the final film width after laminating.
Problems of trimming the excess polymer width away from the metal substrate include issues of reliably tracking the metal edge, damage to the cutting knives by the metal substrate for minor control errors, difficulties with metal edge sensing under a wider film, and the ability to move knives dynamically without damaging the blade or the blade bearing need to be addressed.
Another commercial operating difficulty is matching the length of a film roll to the strip length. Difficulties with length matching cause film yield problems and increase operating costs. It is not economically desirable to discard or recycle partial film rolls. It is also undesirable to stop the laminating process and change film rolls. An operational method must be created to address this issue for good coating economy.
When operating a batch thermal laminating line, it is difficult to completely cover the entire length of a coiled strip. The line must be threaded for each coil, strip tension established, the passline correctly established throughout the line, the film must be inserted into the laminating nip, any film wrinkles eliminated, the correct laminating pressure applied, the line brought up to operational speed, and the correct laminating temperature established. These processing steps, their sequence, and timing must be carefully coordinated and controlled to ensure optimum coating efficiency and yield. Important technical design features and operating methods must be included in the line operation to ensure the greatest operational efficiency.
An important aspect of operating a batch thermal laminating line is to provide for rapid control of the correct metal substrate laminating temperature, especially when establishing the initial film lamination to the metal substrate. Laboratory measurements show that a hot metal strip, at approximately 450° F., cools very rapidly in air. The cooling rate may be 10 to 50° F. per second, depending upon conditions. If the strip preheating is too far from the polymer-metal nip point at a slow line speed, the metal temperature will be too low or unpredictable for reliable laminating. Poor temperature uniformity will directly affect laminating quality and yield. A compensating control method must be utilized.
The use of heated rolls to create the preheated metal temperature may be problematical, particularly if the temperature of the metal strip must be varied. For example, it may be desirable to provide for an initial, higher temperature for lamination until the rolls which press the film onto the strip heat up. A heated roll has a large mass that must be heated and consequently has a poor temperature control response. If the strip temperature must be adjusted, the line speed changes, or the metal substrate temperature changes due to pretreatment changes, it is not feasible to rapidly change the roll temperature. It is important to provide instantaneously adjustable heating and coordinate it with a rapid response, accurate control system.
It is also important that the metal preheating does not affect the surface energy of the metal surface. In laboratory experience, surface pretreatment by a controlled flame provides important adhesive and wet properties to the metal surface. If a heating roll touches the metal surface after surface energy pretreatment, there is a likelihood of adhesion failure. In laboratory experience, sporadic and sparse adhesion defects were diagnosed to be caused by this problem.
Consequently, the design of the production line must consider ways to optimize and maintain the strip as clean as possible when entering the laminating nip. In particular, once the metal strip is pretreated, the metal surface is likely to pick up contamination from any debris on any roll it may touch. The debris may be light oils, dirt, dust, water, finger prints, cleaning residue, various chemicals, etc. Laboratory experience has shown that the pretreated metal surface will eventually remove common debris from processing rolls. However, the laminating quality will be substandard with places of air entrapment and poor adhesion until the debris is gone. The design of the line must consider methods that achieve an immediate, high quality lamination as rapidly as possible.
It is also difficult to accurately monitor the metal temperature just prior to lamination as non-contact sensors in the temperature range below 500° F. are known to be inaccurate and unreliable for metal surfaces such as steel and aluminum. A control method for providing an accurate, reliable laminating temperature feedback must be included in the line design to provide for a high quality laminating process with a high yield.
It is important that the metal substrate surface is properly cleaned sufficiently to achieve complete contact between the film and coil without air entrapment. U.S. Pat. No. 6,200,409 by Tanaka, et al, and U.S. Pat. No. 5,679,200 by Newcomb et al. describe problems with air entrapment between the laminating film and the metal substrate. Laboratory experience indicates that this problem is also clearly related to the surface energy of the metal substrate. The needed metal surface energy for proper film wetting on a thick metal substrate has not been disclosed. The practical ability of the laminating facility to achieve the proper surface energy with pretreatment becomes an important surface specification between the metal supplier and the laminating facility.
It is important to arrange the needed processing steps with a minimum line length. The line is normally threaded by hand and it is highly troublesome to attempt lamination during the threading operation. It is necessary to run pretreatment or heating systems only when there is assurance that the strip will not stop for safety and operational reasons. Consequently, pretreating, preheating, and film laminating is not started until the strip is completely threaded from reel to reel and strip tension is established. This will cause strip lengths at both ends of the coil to be uncoated with an associated yield loss. The yield loss is minimized by designing the line in a compact manner, including only the space and processing steps necessary for lamination. Methods of minimizing metal substrate yield loss become an important operational problem.
It is important to have a way to rapidly cool the film-metal laminate after the reheat step. Three practical methods to control temperature are by forced air, water quench, or contact with a thermally controlled roll. Each type of control has practical operational problems that must be optimized.
The forced air cooling is a relatively slow process, requiring a long section of line. The length of the line increases, thereby causing a higher yield loss. Also, the slower method may cause undesirable crystallinity effects in the film-metal laminate.
The water quench contacts the polymer surface with a fluid that is difficult to completely remove. However, the water system is attractive in that it provides rapid cooling in a short span.
Contacting thermal rolls may have undesirable polymer sticking if the polymer is too hot or the roll surface is damaged.
Also, laboratory observations are that cooling a film-metal laminate utilizing a contact cooling roll can be problematical. A clicking sound can be heard from the cooling roll due to uneven thermal control. The laminate surface simultaneously has uneven crystallization effects.
It is important to prevent damage to the roll surfaces of the processing line during the threading operation. Laboratory experience has shown that bits of rubber and metal from the scratching of rubber covered rolls and steel rolls will affect laminate quality. Operational procedures to ensure a satisfactory operation, free of roll surface damage, need to be incorporated into the production line.
An alternative to the existing coil paint technology is currently being sought due to the environmental problems with paint solvents. Roughly, half of the paint volume used in coil paint lines is solvent that must be evaporated and burned off. Coil paint line drying ovens are required to operate with a negative pressure to ensure that solvents do not escape into the atmosphere. Laminating technology, in connection with thermoplastic films, create no airborne environmental issues. Further, laminating technology holds the promise of lower coating and operating costs.